System and method for controlling an engine that includes low pressure EGR

ABSTRACT

Methods and systems for operating an engine that includes a low pressure EGR passage and a selective reduction catalyst are disclosed. In one example, an actuator is adjusted in response to a NOx mass flow rate in the low pressure EGR passage.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 13/910,818, entitled “SYSTEM AND METHOD FOR CONTROLLING AN ENGINETHAT INCLUDES LOW PRESSURE EGR,” filed on Jun. 5, 2013, the entirecontents of which are hereby incorporated by reference for all purposes.

BACKGROUND/SUMMARY

A turbocharged engine may include high pressure exhaust gasrecirculation (EGR) and low pressure EGR. High pressure EGR may beprovided to an engine by passing exhaust gas from an exhaust system at alocation upstream of a turbocharger turbine to an engine intake systemat a location downstream of a turbocharger compressor. Low pressure EGRmay be provided to an engine by passing exhaust gas from an engineexhaust system at a location downstream of a turbocharger turbine to anengine intake system at a location upstream of the turbochargercompressor. Low pressure EGR may have the benefit of being cooler thanhigh pressure EGR so that engine charge temperature may be reduced. Onthe other hand, by using high pressure EGR, an engine control system mayreduce an EGR mass fraction inducted into a cylinder at a faster rate inresponse to a change in engine load as compared to when low pressure EGRis provided to the engine. Thus, there may be advantages anddisadvantages to using high pressure EGR and low pressure EGR.

The inventors herein have also recognized that high pressure EGR and lowpressure EGR may be comprised of the same or different constituents.Consequently, engine emissions may vary depending on whether highpressure EGR or low pressure EGR is supplied to the engine. Theinventors have addressed the differences between supplying high pressureEGR and low pressure EGR to an engine by developing a method foroperating an engine, comprising: adjusting an actuator in response to aNOx mass flow rate in a low pressure EGR passage between an engineexhaust system and an engine air intake system.

By adjusting an actuator responsive to a NOx mass flow rate in a lowpressure EGR passage, it may be possible to provide a technical resultof adjusting engine NOx emissions to a desirable level. For example, ifEGR is being supplied to the engine with a low NOx mass flow rate, anengine actuator may be adjusted to increase the engine's NOx mass flowoutput and engine fuel economy such that engine's NOx mass flow outputremains below a threshold NOx level. Alternatively, if EGR is suppliedto the engine with a higher NOx mass flow rate, the engine actuator maybe adjusted to decrease the engine's NOx mass flow output. NOx suppliedto the engine via EGR passes through the engine and cannot be reducedduring combustion via adjusting engine operation. However, NOx formedduring combustion of an air-fuel mixture may be adjusted inversely withrespect to NOx supplied to the engine via EGR so that a desired engineNOx level may be provided. Thus, if the desired engine NOx mass flowrate is a constant, and if the NOx flow rate of exhaust gases locateddownstream of a selective catalytic reduction (SCR) catalyst isdecreasing because of higher SCR efficiency, NOx formed in the engine asa result of combustion may be increased without increasing the engine'sNOx mass flow rate since NOx flowing into the engine via EGR isdecreasing.

The present description may provide several advantages. For example, theapproach may allow engine emissions to be maintained at a desired levelwhile engine fuel economy is improved. Additionally, the approach may beuseful for improving the exchange of urea use for vehicle fuel economy.Further, the approach may be useful for improving transient engineemissions.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic depiction of an engine;

FIG. 2 shows a plot of SCR catalyst conversion efficiency versus SCRcatalyst inlet gas temperature;

FIG. 3 is an example flow chart of a method for controlling an enginethat includes high pressure EGR and low pressure EGR; and

FIG. 4 is an example flow chart of a method for adjusting EGR betweenlow pressure and high pressure EGR loops.

DETAILED DESCRIPTION

The present description is related to improving operation of an enginethat includes high pressure EGR and low pressure EGR. Engine operationmay be improved via compensating for changes in NOx levels that mayoccur in low pressure EGR systems due to after treatment operatingconditions. FIG. 1 shows one example of a boosted diesel engine wherethe method of FIG. 3 may adjust engine operation to compensate for NOxin EGR gases. FIG. 2 shows NOx conversion efficiency for a selectivereduction catalyst and it provides insight to the range of NOxconversion efficiency for a SCR.

Referring to FIG. 1, internal combustion engine 10, comprising aplurality of cylinders, one cylinder of which is shown in FIG. 1, iscontrolled by electronic engine controller 12. Engine 10 includescombustion chamber 30 and cylinder walls 32 with piston 36 positionedtherein and connected to crankshaft 40. Combustion chamber 30 is showncommunicating with intake manifold 44 and exhaust manifold 48 viarespective intake valve 52 and exhaust valve 54. Each intake and exhaustvalve may be operated by an intake cam 51 and an exhaust cam 53. Theposition of intake cam 51 may be determined by intake cam sensor 55. Theposition of exhaust cam 53 may be determined by exhaust cam sensor 57.

Fuel injector 66 is shown positioned to inject fuel directly intocombustion chamber 30, which is known to those skilled in the art asdirect injection. Fuel injector 66 delivers fuel in proportion to thepulse width of signal FPW from controller 12. Fuel is delivered to fuelinjector 66 by a fuel system including a fuel tank (not shown), fuelpump (not shown), fuel pump control valve (not shown), and fuel rail(not shown). In addition, a metering valve may be located in or near thefuel rail for closed loop fuel control. A pump metering valve may alsoregulate fuel flow to the fuel pump, thereby reducing fuel pumped to ahigh pressure fuel pump.

Intake manifold 44 is shown communicating with optional electronicthrottle 62 which adjusts a position of throttle plate 64 to control airflow from intake boost chamber 46. Compressor 162 draws air from airintake 42 to supply boost chamber 46. Exhaust gases spin turbine 164which is coupled to compressor 162 via shaft 161. In some examples, acharge air cooler may be provided. Compressor speed may be adjusted viaadjusting a position of variable vane control 72 or compressor bypassvalve 158. Variable vane control 72 adjusts a position of variablegeometry turbine vanes. Exhaust gases can pass through turbine 164supplying less energy to rotate turbine 164 when turbine vanes are in anopen position. Exhaust gases can pass through turbine 164 and impartincreased force on turbine 164 when turbine vanes are in a closedposition. Compressor bypass valve 158 allows compressed air at theoutlet of compressor 162 to be returned to the input of compressor 162.In this way, the efficiency of compressor 162 may be reduced so as toaffect the flow of compressor 162 and reduce intake manifold pressure.

Combustion is initiated in combustion chamber 30 when fuel ignites viacompression ignition as piston 36 approaches top-dead-center compressionstroke. In some examples, a universal Exhaust Gas Oxygen (UEGO) sensor126 may be coupled to exhaust manifold 48 upstream of emissions device70. In one example, emissions device 70 is a selective catalyticreduction (SCR) catalyst. Alternatively, emissions device 70 is a leanNOx trap (LNT). Further, in some examples, the UEGO sensor 126 may be aNOx sensor that has both NOx and oxygen sensing elements. NOx sensor 129outputs a voltage that is proportional to the concentration of NOxupstream of turbine 164. Alternatively, sensor 126 may be positioneddownstream of turbine 164 and upstream of emissions device 70. NOxsensor 127 samples tailpipe NOx downstream of emissions device 70.

NOx concentration and NOx mass flow rate may be determined at locations141-147. Location 141 is in engine exhaust manifold 48 upstream of highpressure EGR passage 71. Location 142 is downstream of turbine 164 andupstream of emissions device 70. Location 143 is at the outlet ofemissions device 70 and upstream of low pressure EGR passage 81.Location 145 is in low pressure EGR passage 81. Location 144 is at alocation downstream of emissions device 70 and downstream of EGR valve80 in the direction of exhaust flow to the tail pipe 150. Location 146is in high pressure EGR passage 76.

At lower engine temperatures glow plug 68 may convert electrical energyinto thermal energy so as to raise a temperature in combustion chamber30. By raising temperature of combustion chamber 30, it may be easier toignite a cylinder air-fuel mixture via compression ignition.

As mentioned, in one example, emissions device 70 can include SCRcatalyst bricks or a LNT. In another example, multiple emission controldevices, each with multiple bricks, can be used. Emissions device 70 caninclude an oxidation catalyst in one example. In other examples, theemissions device may include a lean NOx trap followed by a selectivecatalyst reduction (SCR), and/or a diesel particulate filter (DPF). Ureamay be injected upstream of SCR catalyst 70 via urea injector 90. Ureainjector 90 receives urea from urea tank 91. Level sensor 93 senses theamount of urea stored in urea tank 91.

Low pressure exhaust gas recirculation (EGR) may be provided to theengine via EGR valve 80. EGR valve 80 is a two-way valve that closes orallows exhaust gas to flow from downstream of emissions device 70 to alocation in the engine air intake system upstream of compressor 162. Insome examples, low pressure EGR passage may include a throttle betweenintake passage 42 and low pressure EGR valve 80 or in tailpipe 150 tocreate a pressure differential.

High pressure EGR may be provided to the engine via high pressure EGRvalve 75 and high pressure EGR passage 76. High pressure EGR may flowfrom exhaust manifold 48 to a location downstream of throttle 62 whenhigh pressure EGR valve 75 is open and when pressure in exhaust manifold48 is greater than pressure in intake manifold 44. High pressure EGRpassage 76 and low pressure EGR passage 81 may include an EGR cooler insome examples.

Controller 12 is shown in FIG. 1 as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, read-onlymemory 106, random access memory 108, keep alive memory 110, and aconventional data bus. Controller 12 is shown receiving various signalsfrom sensors coupled to engine 10, in addition to those signalspreviously discussed, including: engine coolant temperature (ECT) fromtemperature sensor 112 coupled to cooling sleeve 114; a position sensor134 coupled to an accelerator pedal 130 for sensing accelerator positionadjusted by foot 132; a measurement of engine manifold pressure (MAP)from pressure sensor 121 coupled to intake manifold 44; upstream exhaustpressure from pressure sensor 151; downstream, exhaust pressure frompressure sensor 152; boost pressure from pressure sensor 122 exhaust gasoxygen concentration from oxygen sensor 126; an engine position sensorfrom a Hall effect sensor 118 sensing crankshaft 40 position; ameasurement of air mass entering the engine from sensor 120 (e.g., a hotwire air flow meter); and a measurement of throttle position from sensor58. Barometric pressure may also be sensed (sensor not shown) forprocessing by controller 12. In a preferred aspect of the presentdescription, engine position sensor 118 produces a predetermined numberof equally spaced pulses every revolution of the crankshaft from whichengine speed (RPM) can be determined.

During operation, each cylinder within engine 10 typically undergoes afour stroke cycle: the cycle includes the intake stroke, compressionstroke, expansion stroke, and exhaust stroke. During the intake stroke,generally, the exhaust valve 54 closes and intake valve 52 opens. Air isintroduced into combustion chamber 30 via intake manifold 44, and piston36 moves to the bottom of the cylinder so as to increase the volumewithin combustion chamber 30. The position at which piston 36 is nearthe bottom of the cylinder and at the end of its stroke (e.g. whencombustion chamber 30 is at its largest volume) is typically referred toby those of skill in the art as bottom dead center (BDC). During thecompression stroke, intake valve 52 and exhaust valve 54 are closed.Piston 36 moves toward the cylinder head so as to compress the airwithin combustion chamber 30. The point at which piston 36 is at the endof its stroke and closest to the cylinder head (e.g. when combustionchamber 30 is at its smallest volume) is typically referred to by thoseof skill in the art as top dead center (TDC). In a process hereinafterreferred to as injection, fuel is introduced into the combustionchamber. In some examples, fuel may be injected to a cylinder aplurality of times during a single cylinder cycle. In a processhereinafter referred to as ignition, the injected fuel is ignited bycompression ignition resulting in combustion. During the expansionstroke, the expanding gases push piston 36 back toward BDC. Crankshaft40 converts piston movement into a rotational torque of the rotaryshaft. Finally, during the exhaust stroke, the exhaust valve 54 opens torelease the combusted air-fuel mixture to exhaust manifold 48 and thepiston returns to TDC.

Note that the above is described merely as an example, and that intakeand exhaust valve opening and/or closing timings may vary, such as toprovide positive or negative valve overlap, late intake valve closing,or various other examples. Further, in some examples a two-stroke cyclemay be used rather than a four-stroke cycle.

Thus, the system of FIG. 1 provides for an engine system, comprising: anengine including a turbocharger, a high pressure EGR passage, a lowpressure EGR passage, an air intake, and an exhaust manifold; an exhaustsystem coupled to the exhaust manifold and including a selectivereduction catalyst; and a controller including executable instructionsstored in non-transitory memory, the executable instructions adjustingan actuator in response to a NOx concentration of an EGR mass flowingthrough the low pressure EGR passage. The engine system includes wherethe low pressure EGR passage provides fluidic communication between theair intake passage and the exhaust system at a location downstream ofthe selective reduction catalyst.

In some examples, the engine system includes where the actuator is afuel injector, and further comprising additional instructions formaintaining a desired EGR flow rate while a NOx concentration in the lowpressure EGR passage varies, and adjusting a start of fuel injectiontiming for the fuel injector in response to the NOx concentration in thelow pressure EGR passage. The engine system further comprises a NOxsensor positioned downstream of the selective reduction catalyst. Theengine system includes where an output of the NOx sensor isrepresentative of the NOx concentration of the EGR mass flowing throughthe low pressure EGR passage. The engine system includes where thecontroller includes further instructions for selectively providing EGRto the engine via the high pressure EGR passage and the engine and thelow pressure EGR passage.

Referring now to FIG. 2, a plot of SCR catalyst conversion efficiencyversus SCR catalyst inlet gas temperature is shown. Plot 200 representsan example of NOx conversion efficiency for emissions device 70 ofFIG. 1. The Y axis represents NOx conversion efficiency in percentage.The X axis represents SCR inlet gas temperature in degrees C.

SCR efficiency curve 202 shows that emissions control device 70 has lowNOx conversion efficiency at temperatures below 150° C. For example, NOxconversion efficiency at 150° C. is about 40 percent and lower for lowerinlet gas temperatures. It may be observed that NOx conversionefficiency increases rapidly and reaches about 90 percent at about 185°C. Further, NOx conversion efficiency of emissions control device 70increases slowly at temperatures above 185° C. and approaches 100percent efficiency. Near 390° C., NOx conversion efficiency is reducedback to about 90 percent. NOx conversion efficiency continues todecrease as SCR inlet temperature continues to increase.

Thus, it may be observed that the concentration of NOx in low pressureEGR gases may vary even if engine output NOx is constant because SCRefficiency may vary. Further, as described in more detail with regard tothe method of FIG. 3, engine output NOx (e.g., NOx produced duringcombustion and NOx recirculated back to the engine via exhaust gas) maybe affected by the amount of NOx supplied to the engine via a lowpressure EGR passage. Therefore, it may be beneficial to consider engineoutput NOx concentration or NOx mass flow rate and SCR efficiency whenestimating the amount of NOx an engine produces when receiving lowpressure EGR.

Referring now to FIG. 3, a method for operating an engine having low andhigh pressure EGR passages is shown. In on example, the system of FIG. 1may be operated according to the method of FIG. 3. Further, the methodof FIG. 3 may be incorporated into controller 12 of FIG. 1 viaexecutable instructions stored in non-transitory memory.

At 302, method 300 determines SCR efficiency and the NOx concentrationof exhaust gases downstream of a SCR. The SCR may be positioned in anexhaust system as shown in FIG. 1. In one example, SCR efficiency may bedetermined via subtracting a NOx concentration at a location downstreamof the SCR from a NOx concentration upstream of the SCR and dividing theresult by the NOx concentration at the location upstream of the SCR.Alternatively, SCR efficiency may be estimated based on SCR temperature,SCR age, NH₃ storage, and space velocity of the SCR. Method 300 proceedsto 304 after the SCR efficiency is determined.

At 304, method 300 judges whether a time since engine stop (e.g., enginerunning time) is greater than a threshold time or if SCR efficiency isgreater than a threshold SCR efficiency or if a NOx concentrationdownstream of a SCR is less than a threshold NOx concentration. If timesince engine stop is greater than a threshold amount of time, or if SCRefficiency is greater than a threshold SCR efficiency, or if a NOxconcentration downstream of a SCR is less than a threshold NOxconcentration, the answer is yes and method 300 proceeds to 308.Otherwise, the answer is no and method 300 proceeds to 306.

At 306, method 300 operates a high pressure EGR loop responsive tooperating conditions. In one example, the high pressure EGR loop isoperated in response to engine speed and load. Further, the highpressure EGR loop may be operated in response to engine temperature. Ifengine speed and load reach conditions where the high pressure EGR loopis operated, the high pressure EGR valve is opened and high pressure EGRis allowed to flow from a location in the exhaust system upstream of aturbine to an engine intake manifold. Method 300 proceeds to exit afterthe high pressure EGR loop is selectively operated.

At 308, method 300 judges whether or not the low pressure EGR loop is inoperation. The low pressure EGR loop is being operated when EGR ispassing through the low pressure EGR passage to the engine air intake.The low pressure EGR loop may include the engine, the low pressure EGRpassage, the engine air intake, and the exhaust system. If the lowpressure EGR loop is operated the answer is yes and method 300 proceedsto 310. Otherwise, the answer is no and method 300 proceeds to 306.

At 310, method 300 activates low pressure and high pressure EGR loopswithout necessarily operating both the high pressure EGR loop and thelow pressure EGR loop responsive to operating conditions. In oneexample, the high pressure EGR loop is activated by allowing opening ofa high pressure EGR valve, and it allows high pressure EGR to flow froma location in an exhaust system upstream of a turbine to the engineintake manifold. The low pressure EGR loop is activated by allowingopening of a low pressure EGR valve, and it allows low pressure EGR toflow from a location in an exhaust system downstream of a turbine to theengine intake system upstream of a compressor. The low pressure and highpressure EGR systems may be selectively operated after being activatedin response to engine and vehicle operating conditions. Method 300proceeds to 312 after the low and high pressure EGR loops are activated.

At 312, method 300 estimates NOx concentration at a plurality oflocations in the exhaust system shown in FIG. 1. Further, method 300determines the total mass flow, NOx concentrations, and NOx mass flowrates at selected locations in the exhaust system shown in FIG. 1.

The mass flow rate of NOx at location 141 is given by:{dot over (m)} ₁₄₁ ={dot over (m)} _(a) +{dot over (m)} _(f) +{dot over(m)} ₁₄₆ +{dot over (m)} ₁₄₅ (total mass flow at location 141){dot over (m)} _(NOx,141) ={dot over (m)} ₁₄₁[NOx] ₁₄₁ C ₀ (mass flowrate of NOx at location 141){dot over (m)} _(NOx,141) ={dot over (m)} _(NOx,eng) +{dot over (m)}_(NOx,146) +{dot over (m)} _(NOx,145)Where {dot over (m)}₁₄₁ is a total mass flow rate at position 141 inFIG. 1, {dot over (m)}_(a) is a mass flow rate of air entering theengine and it may be determined via a mass air flow sensor, {dot over(m)}_(f) is a mass of fuel entering the engine and it may be determinedvia fuel injector pulse widths, {dot over (m)}₁₄₅ is a total mass flowrate at position 145 in FIG. 1, and where {dot over (m)}₁₄₆ is a totalmass flow rate at position 146 in FIG. 1. [NOx]₁₄₁ represents aconcentration of NOx at location 141 and C₀ represents a unit conversionfactor. The mass flow rate of NOx at location 141 (engine output) isgiven by {dot over (m)}_(NOx,141) and it is a sum of NOx mass flowformed in the engine {dot over (m)}_(NOx,eng), high pressure passage NOxmass flow {dot over (m)}_(NOx,146), and low pressure passage NOx massflow {dot over (m)}_(NOx,145).

The mass flow rate of NOx at location 142 is given by:{dot over (m)} ₁₄₂ ={dot over (m)} _(a) +{dot over (m)} _(f) +{dot over(m)} ₁₄₅ (total mass flow at location 142){dot over (m)} _(NOx,142) ={dot over (m)} ₁₄₂[NOx] ₁₄₂ C ₀ (mass flowrate of NOx at location 142){dot over (m)} _(NOx,142) ={dot over (m)} _(NOx,eng) +{dot over (m)}_(NOx,145)[NOx] ₁₄₂=[NOx] ₁₄₁Where {dot over (m)}₁₄₂ is the total mass flow rate at location 142,[NOx]₁₄₂ represents a concentration of NOx at location 142 and C₀represents a unit conversion factor. The remaining parameters are aspreviously described.

The mass flow rate of NOx at location 143 is given by:

${\overset{.}{m}}_{143} = {{{\overset{.}{m}}_{a} + {\overset{.}{m}}_{f} + {\overset{.}{m}}_{145}} = {{\overset{.}{m}}_{142}\mspace{14mu}\left( {{total}\mspace{14mu}{mass}\mspace{14mu}{flow}\mspace{14mu}{at}\mspace{14mu}{location}\mspace{14mu} 143} \right)}}$$\mspace{79mu}{{\overset{.}{m}}_{{NOx},143} = {{{\overset{.}{m}}_{142}\lbrack{NOx}\rbrack}_{142}\left( {1 - \frac{\eta_{NOx}}{100}} \right)C_{0}}}$      (mass  flow  rate  of  NOx  at  location  143)  ${\overset{.}{m}}_{{NOx},143} = {{{\overset{.}{m}}_{{NOx}\; 142}\left( {1 - \frac{\eta_{NOx}}{100}} \right)} = {\left( {{\overset{.}{m}}_{{NOx},{eng}} + {\overset{.}{m}}_{{NOx},145}} \right) \cdot \left( {1 - \frac{\eta_{NOx}}{100}} \right)}}$Where {dot over (m)}₁₄₃ is the total mass flow rate at location 143,{dot over (m)}_(NOx,143) is the mass flow rate of NOx at location 143,η_(NOx) is the NOx conversion efficiency of the SCR (e.g., 70 in FIG.1). The other parameters are as previously described.

The mass flow rate of NOx at location 144 is given by:

${\overset{.}{m}}_{144} = {{\overset{.}{m}}_{a} + {\overset{.}{m}}_{f}}$${\overset{.}{m}}_{{NOx},144} = {{\overset{.}{m}}_{{NOx},143} - {\overset{.}{m}}_{{NOx},145}}$${\overset{.}{m}}_{{NOx},144} = {{{\overset{.}{m}}_{{NOx},{eng}}\left( {1 - \frac{\eta_{NOx}}{100}} \right)} - {{\overset{.}{m}}_{{NOx},145}\left( \frac{\eta_{NOx}}{100} \right)}}$Where {dot over (m)}₁₄₄ is the total mass flow rate at location 144 andwhere {dot over (m)}_(NOx,144) is the mass flow rate of NOx at location144. The other parameters are as previously described.

The mass flow rate of NOx at location 145 is given by:

${\overset{.}{m}}_{{NOx},145} = {\frac{{\overset{.}{m}}_{145}}{{\overset{.}{m}}_{143}}{\overset{.}{m}}_{{NOx}\; 143}}$${\overset{.}{m}}_{{NOx},145} = {{{\overset{.}{m}}_{145}\lbrack{NOx}\rbrack}_{142}{C_{O}\left( {1 - \frac{\eta_{NOx}}{100}} \right)}}$${{{\overset{.}{m}}_{142}\lbrack{NOx}\rbrack}_{142}C_{O}} = {{\overset{.}{m}}_{{NOx},{eng}} + {{{\overset{.}{m}}_{145}\lbrack{NOx}\rbrack}_{142}{C_{O}\left( {1 - \frac{\eta_{NOx}}{100}} \right)}}}$${\overset{.}{m}}_{{NOx},{eng}} = {{{\left\lbrack {{\overset{.}{m}}_{142} - {{\overset{.}{m}}_{145}\left( {1 - \frac{\eta_{NOx}}{100}} \right)}} \right\rbrack\lbrack{NOx}\rbrack}_{142}{C_{O\;}\lbrack{NOx}\rbrack}_{142}} = \frac{{\overset{.}{m}}_{{NOx},{eng}}}{C_{O}\left( {{\overset{.}{m}}_{a} + {\overset{.}{m}}_{f} + {{\overset{.}{m}}_{145}\frac{\eta_{NOx}}{100}}} \right)}}$Where {dot over (m)}_(NOx,145) is the NOx mass flow rate at location145, [NOx]₁₄₂ is the concentration of NOx at location 142, and where theother parameters are as previously described. The NOx mass flow rate atlocation 145 is given by

${{\overset{.}{m}}_{145}\lbrack{NOx}\rbrack}_{142}{{C_{0}\left( {1 - \frac{\eta}{100}} \right)}.}$

The inventors show one way to estimate the mass flow rate of NOx into anSCR catalyst.

Case 1: after treatment is 100% efficient{dot over (m)} _(NOx,145)=0{dot over (m)} _(Nox147) ={dot over (m)} _(NOx,eng){dot over (m)} _(NOx147)=({dot over (m)} _(a) +{dot over (m)} _(f) +{dotover (m)} ₁₄₅)·C ₀·[NOx] _(FG1)Where {dot over (m)}_(NOx,145) is the NOx mass flow rate at location145, where {dot over (m)}_(NOx,147) is the NOx mass flow rate atlocation 147, where {dot over (m)}_(NOx,eng) is the NOx mass flow rateat location 141 or output from the engine, where {dot over (m)}_(a) is amass flow rate of air entering the engine, where {dot over (m)}_(f) is amass of fuel entering the engine, where {dot over (m)}₁₄₅ is a totalmass flow rate at position 145 in FIG. 1, where C₀ is a unit conversionfactor, and where [NOx]_(FG1) is engine output NOx concentration forcase 1. Case 1 shows one extreme where the SCR converts all NOx enteringthe SCR to N₂ and H₂O and where the amount of NOx entering the SCR isequivalent to the amount of NOx produced by the engine.

Case 2: after treatment is 0% efficient{dot over (m)} _(NOx147) ={dot over (m)} _(NOx,eng) +{dot over (m)}_(NOx,145){dot over (m)} _(NOx,145) ={dot over (m)} ₁₄₅[NOx] _(FG2) ·C ₀{dot over (m)} _(NOx,147) ={dot over (m)} _(NOx,eng) +{dot over (m)}₁₄₅[NOx] _(FG2) ·C ₀{dot over (m)} _(NOx,147) ={dot over (m)} _(a) +{dot over (m)} _(f)+{dot over (m)} ₁₄₅[NOx] _(FG1) ·C ₀ +{dot over (m)} ₁₄₅[NOx] _(FG2) ·C₀{dot over (m)} _(NOx,147) ={dot over (m)} _(a) +{dot over (m)} _(f)+{dot over (m)} ₁₄₅[NOx] _(FG2) ·C ₀{dot over (m)} _(NOx,144) ={dot over (m)} _(NOx,eng){dot over (m)} _(NOx,eng)=({dot over (m)} _(a) +{dot over (m)}_(f))[NOx] _(FG2) ·C ₀Where the parameters of case 2 are the same as for case 1 and where[NOx]_(FG2) is engine output NOx concentration for case 2. Case 2 showsthe other extreme where the SCR converts no NOx entering the SCR, andthe NOx entering the SCR is the amount of NOx produced by the engineplus the NOx recirculated through the low pressure EGR passage.

NOx production in the engine is equivalent between case 1 and case 2since the boundary conditions for combustion between the two cases isequivalent. Therefore, the following equation holds:{dot over (m)} _(NOx,147)=({dot over (m)} _(a) +{dot over (m)} _(f)+{dot over (m)} ₁₄₅)[NOx] _(FG1) ·C ₀=({dot over (m)} _(a) +{dot over(m)} _(f))[NOx] _(FG2) ·C ₀

As a result, an estimate of a concentration of engine output NOx whilean SCR is active may be determined from a concentration of engine outputNOx when the SCR is inactive. In particular, [NOx]_(FG2) is empiricallydetermined and stored in memory as a table or function. The table orfunction may be indexed by speed and load as well as other parameters.Variables {dot over (m)}_(a) and {dot over (m)}_(f) are know from fuelinjection timing and intake air measurements. Additionally, {dot over(m)}₁₄₅ may be determined based on a pressure differential across lowpressure EGR valve 80 and low pressure EGR valve position. Unknownvariable [NOx]_(FG1) may be determined from the know variables. Once[NOx]_(FG1) is determined, it may be the basis for adjusting ureainjection, EGR, fuel pressure, boost pressure, and other actuators.Thus, the NOx concentration in the high pressure EGR passage may bebased on estimated SCR efficiency or a known NOx concentration when theSCR is operating at less than a threshold efficiency. Further, the NOxconcentration entering the SCR may be estimated without a NOx sensorpositioned upstream of emissions device 70 in the system of FIG. 1.

Thus, a method for operating an engine is provided, comprising:adjusting an actuator in response to a NOx mass flow rate into a NOxcatalyst (e.g., LNT or SCR) when the NOx catalyst is operating at anefficiency greater than a first threshold efficiency during a firstcondition, the NOx mass flow rate into the NOx catalyst based ontailpipe NOx mass flow rate when the NOx catalyst is operating at anefficiency of less a second threshold efficiency. The method includeswhere the NOx catalyst is a SCR or a LNT. The method includes where theactuator is a urea injector or an EGR valve. The method also includeswhere the actuator is a fuel pump. The method includes where the firstthreshold efficiency is greater than 50% and where the second thresholdefficiency is less than 25%. In some examples, the method furthercomprising adjusting the actuator in response to the an estimate of NOxflow rate into the NOx catalyst during a second condition, where theestimate of NOx flow rate into the NOx catalyst is based on one of NOxcatalyst efficiency, mass flow rate of NOx through low pressure EGRloop, or NOx concentration in the low pressure EGR loop. The first andsecond conditions may be different SCR or LNT operating conditions. Suchoperation may be used in conjunction with adjusting an actuator inresponse to a concentration of NOx in a low pressure and/or highpressure EGR passage.

Returning to FIG. 3, method 300 proceeds to 314 after the total massflow rate, NOx flow rate, and NOx concentration at each location isdetermined.

At 314, method 300 judges whether or not to adjust EGR mass flow rate inresponse to low pressure EGR loop NOx concentration. For example, method300 may adjust the EGR mass flow rate based on NOx concentration so thata desired NOx flow rate in the low pressure EGR loop may be provided tothe engine. On the other hand, the total EGR mass flow rate may beadjusted to follow a desired EGR mass flow rate so that a desired EGRmass flow rate is provided irrespective of the NOx concentration in thelow pressure EGR loop. Thus, in this example, the NOx flow rate may beallowed to vary. In one example, method 300 selects to adjust the EGRmass flow rate through the low pressure EGR passage in response to thelow pressure EGR loop NOx concentration when SCR efficiency is less thanthreshold SCR efficiency so that tailpipe NOx may be controlled. If SCRefficiency is greater than the threshold efficiency, the EGR mass flowrate is adjusted to follow the desired EGR flow rate and an actuator isadjusted to control tailpipe NOx. If method 300 judges that the EGR massis to be adjusted in response to the NOx concentration in the lowpressure EGR loop, the answer is yes and method 300 proceeds to 330.Otherwise, the answer is no and method 300 proceeds to 320.

At 330, method 300 adjusts the total EGR mass flow rate based on a NOxconcentration in the low pressure EGR passage. The total EGR mass flowrate may be comprised of flow through the high and low pressure loops,Thus, high pressure loop EGR may be adjusted based on NOx concentrationin the low pressure EGR loop. For example, if NOx concentration in thelow pressure EGR loop is low due to high SCR efficiency, the low and/orhigh pressure EGR mass flow rate may be decreased. Alternatively, if NOxconcentration in the low pressure EGR loop is high due to high SCRefficiency, the low and/or high pressure EGR mass flow rate may beincreased. If the NOx concentration in the low pressure EGR loop isincreasing or at a high level, the EGR mass flowing through the lowpressure EGR passage or loop may be reduced to maintain a NOx flow ratethrough the low pressure EGR passage. If the NOx concentration in thelow pressure EGR loop is decreasing, the EGR mass flowing through thelow pressure EGR passage may be increased to maintain a NOx flow ratethrough the low pressure EGR passage. The EGR mass flow rate in the lowpressure EGR loop may be increased via increasing an opening amount ofthe low pressure EGR valve. The EGR mass flow rate in the low pressureEGR loop may be decreased via decreasing an opening amount of the lowpressure EGR valve. Method 300 proceeds to 332 after the EGR mass flowrate is adjusted.

At 332, method 300 adjusts an actuator in response to NOx concentrationand/or NOx mass flow rate into the low pressure EGR loop. In oneexample, the actuator may be a fuel injector. The opening time of thefuel injector may be advanced when the NOx concentration decreasesand/or when the NOx mass flow rate decreases. By advancing fuel injectoropening timing when NOx concentration and/or NOx mass flow ratedecreases, the NOx created during combustion may be increased slightlyto offset the NOx concentration and/or NOx mass flow rate decreaseflowing through the engine via the low pressure EGR loop. Further,advancing start of fuel injection opening timing may increase vehiclefuel economy via increasing engine torque. Thus, if the SCR is operatingefficiently, less EGR may be provided to the engine via the low pressureEGR passage so that engine operation may be adjusted to improve engineperformance. If instead, the NOx concentration and/or NOx flow rate isincreasing, the start of injector opening timing may be retarded so thatNOx produced in the engine during combustion may be reduced. In thisway, injector timing may be adjusted to compensate for changes in NOxconcentration and/or NOx flow in the low pressure EGR passage.

In another example, the actuator may be a urea injector. If a NOxconcentration in the low pressure EGR passage or a NOx flow rate in thelow pressure EGR passage is greater than is desired, the urea injectormay be opened further to provide additional urea to the SCR. Similarly,if a NOx concentration in the low pressure EGR passage or a NOx flowrate in the low pressure EGR passage is less than is desired, the ureainjector may be closed further to provide less urea to the SCR.

In another example, the actuator may be a fuel pump. If a NOxconcentration in the low pressure EGR passage or a NOx flow rate in thelow pressure EGR passage is greater than is desired, the fuel pumpoutput pressure may be increased as EGR flow to the engine increases toreduce NOx produced during combustion and reduce particulate emissions.Similarly, if a NOx concentration in the low pressure EGR passage or aNOx flow rate in the low pressure EGR passage is less than is desired,the fuel pump pressure may be decreased as EGR flow to the engine isreduced to engine fuel economy.

In another example, the actuator may be a low pressure EGR valve. If aNOx concentration in the low pressure EGR passage or a NOx flow rate inthe low pressure EGR passage is greater than is desired, the lowpressure EGR valve opening amount may be reduced to lower the amount ofNOx flowing back into the engine. Similarly, if a NOx concentration inthe low pressure EGR passage or a NOx flow rate in the low pressure EGRpassage is less than is desired, the low pressure EGR valve openingamount may be increased to raise the amount of NOx flowing back to theengine. Method 300 proceeds to 334 after the actuator is adjusted.

At 334, method 300 adjusts an actuator in response to the NOx mass flowrate into the SCR. In one example, the actuator is a urea injector andan amount of injected urea is increased in response to an increase inNOx flowing into the SCR. Method 300 proceeds to exit after the actuatoris adjusted.

It should be mentioned that any combination of actuators mentioned at332 may be adjusted in response to NOx concentration and/or NOx flowrate in the low pressure EGR passage.

At 320, method 300 adjusts low pressure passage EGR flow based on adesired low pressure EGR flow. Additionally, an actuator is adjusted inresponse to NOx concentration and/or NOx flow rate in the low pressureEGR loop. For example, if the desired EGR flow rate through the lowpressure EGR passage is a constant flow rate, an actuator is adjusted tocompensate for the NOx concentration and/or NOx mass flow rate in thelow pressure EGR passage. In particular, if the NOx concentration and/orNOx flow rate increases through the low pressure EGR passage, start ofinjection timing for a fuel injector is retarded to reduce the amount ofNOx produced by the engine during combustion. As a result, when NOxflows through the low pressure EGR passage and combines with NOxproduced during combustion, the total amount of NOx is reduced ascompared to if no actuator adjustment occurs. Conversely, if the NOxconcentration and/or NOx flow rate decreases through the low pressureEGR passage, start of injection timing for a fuel injector is advancedto induce the amount of NOx produced by the engine during combustion andincrease engine torque and fuel economy. Therefore, when less NOx flowsthrough the low pressure EGR passage and combines with NOx producedduring combustion, the total amount of NOx may be maintained withincreased fuel efficiency.

In other examples, the actuator may be a fuel pump. The fuel pump outputpressure may be decreased in response to an increase in NOxconcentration and/or NOx mass flow through the low pressure EGR passageso as to reduce the amount of NOx produced during combustion so that thetotal amount of NOx passing through the engine is maintained at adesired level. Similarly, fuel pump output pressure may be increased inresponse to a decrease in NOx concentration and/or NOx mass flow throughthe low pressure EGR passage so as reduce engine soot production.

In another example, the actuator may be a low pressure EGR valve. Thelow pressure EGR valve opening amount may be increased in response to anincrease in NOx concentration and/or NOx mass flow through the lowpressure EGR passage so as to reduce the amount of NOx produced duringcombustion. Similarly, low pressure EGR valve opening amount may bedecreased in response to a reduction in NOx concentration and/or NOxmass flow through the low pressure EGR passage so as to increase enginefuel efficiency while maintaining NOx flow rate into the SCR catalyst.

In another example, the actuator may be a waste gate or a variablegeometry turbine (VGT) actuator for adjusting boost pressure (e.g.,intake manifold pressure). The waste gate opening amount may bedecreased in response to a reduction in EGR flow or NOx concentrationand/or NOx mass flow through the low pressure EGR passage being reducedso as to improve engine fuel efficiency. Similarly, waste gate openingamount may be increased in response to an increase in NOx concentrationand/or NOx mass flow through the low pressure EGR passage so as tomaintain an engine air-fuel ratio in the presence if a higher EGR massflow rate used to reduce NOx produced by the engine during combustion.

In still another example, the actuator may be a urea injector foradjusting urea supplied to a SCR. The urea injector opening time may beincreased in response to an increase in NOx concentration and/or NOxmass flow through the low pressure EGR passage so as to increase SCRefficiency and reduce NOx exiting the SCR. Similarly, the urea injectoropening timing may be decreased in response to a reduction in NOxconcentration and/or NOx mass flow through the low pressure EGR passageso as to decrease urea consumption. Method 300 proceeds to exit afterthe EGR flow rate and actuator are adjusted.

Thus, the method of FIG. 3 provides for a method for operating anengine, comprising: adjusting an actuator in response to a NOx mass flowrate in a low pressure EGR passage between an engine exhaust system andan engine air intake system. The method includes where the actuator is aurea injector, and further comprising decreasing an amount of ureainjected to the engine exhaust system as the NOx mass flow ratedecreases.

In some examples, the method includes where the actuator is a fuelinjector, and further comprising advancing a start of fuel injectiontiming in response to a reduction in NOx mass flow rate in the lowpressure EGR passage. The method includes where the actuator is an EGRvalve, and further comprising reducing an amount of EGR supplied to theengine in response to a reduction in NOx mass flow rate in the lowpressure EGR passage. The method includes where the actuator is a fuelpump, and further comprising reducing fuel pump output pressure inresponse to a reduction in NOx mass flow rate in the low pressure EGRpassage. The method includes where the actuator is a turbocharger wastegate or VGT, and further comprising reducing boost pressure in responseto a reduction in NOx mass flow rate in the low pressure EGR passage.The method includes where the NOx mass flow rate is estimated based on aconcentration of NOx downstream of a SCR catalyst and a mass flow ratein the low pressure EGR passage.

The method of FIG. 3 also provide for a method for operating an engine,comprising: selecting between delivering EGR to an engine via a highpressure EGR passage and a low pressure EGR passage in response to aconcentration of NOx in an exhaust passage location downstream of an SCRcatalyst. The method includes where the high pressure EGR passage isselected in response to a concentration of NOx in the exhaust passagelocation downstream of the SCR catalyst being higher than a thresholdNOx concentration, and where the low pressure EGR passage is selected inresponse to the concentration of NOx in the exhaust passage locationdownstream of the SCR catalyst being less than the threshold NOxconcentration. The method further comprises adjusting an actuator inresponse to NOx mass flow rate in the low pressure EGR passage.

In some examples, the method includes where the actuator is a ureainjector. The method includes where the actuator is a fuel injector, andwhere the fuel injector start of injection timing during a cylindercycle is advanced as a concentration of NOx in the low pressure EGRpassage decreases. The method further comprises injecting an amount ofurea to the SCR catalyst in response to a NOx concentration of exhaustgas entering the SCR catalyst. The method further comprises adjustingEGR mass flow rate in response to a concentration of NOx in the lowpressure EGR passage.

Referring now to FIG. 4, a method for adjusting EGR between low pressureand high pressure EGR loops is shown. In on example, the system of FIG.1 may be operated according to the method of FIG. 4. Further, the methodof FIG. 4 may be incorporated into controller 12 of FIG. 1 viaexecutable instructions stored in non-transitory memory.

At 402, method 400 determines a desired EGR amount or EGR flow rate forthe engine. In one example, the desired EGR amount is empiricallydetermined and stored in memory locations that are indexed based on theengine's present speed and load (e.g., torque). Method 400 proceeds to404 after the desired EGR amount is determined.

At 404, method 400 determines the relative amounts of EGR to be suppliedvia the low pressure EGR loop and the high pressure EGR loop. In oneexample, each of the low pressure EGR loop and the high pressure EGRloop are assigned a fraction of the desired EGR amount. The sum of EGRsupplied in by the low pressure EGR loop and the EGR supplied by thehigh pressure loop is equal to the desired EGR amount. The fractions ofthe total EGR amount that are assigned to the low pressure EGR loop andthe high pressure EGR loop may be based on engine speed and load. Forexample, at an engine speed of 2000 RPM and 0.3 load, 60% of the totaldesired EGR mass flow rate may be provided via the high pressure EGRloop and 40% of the total desired EGR mass flow rate may be provided viathe low pressure EGR loop. Further, the fractions may be adjusted basedon engine temperature, SCR temperature, and other operating conditions.Method 400 proceeds to 406 after low pressure and high pressure EGRrelative amount are determined.

At 406, method 400 determines NOx concentration and/or NOx flow rate ofexhaust gases in the high pressure and low pressure EGR loops. In oneexample, the NOx concentrations in the high pressure EGR loop may bedetermined via a NOx sensor such as NOx sensors 126 or 129 shown inFIG. 1. For example, the NOx concentration at NOx sensor 129 is NOxconcentration in high pressure EGR passage 76 and NOx concentration atNOx sensor 127 is NOx concentration in low pressure EGR passage 81. Inone example, the NOx mass flow rate in high pressure EGR passage 76 isdetermined by a pressure differential between exhaust manifold 48 andintake manifold 44 as determined via pressure sensors 121 and 129. Thepressure differential is a basis for indexing an empirically determinedfunction that describes flow through high pressure EGR valve 75. Themass flow through high pressure EGR passage 76 is multiplied by the NOxconcentration in high pressure EGR passage 76 to provide a NOx mass flowrate through high pressure EGR passage 76. Similarly, the NOx mass flowrate in low pressure EGR passage 81 is determined by a pressuredifferential between intake passage 42 and tail pipe 150 as determinedvia barometric pressure and pressure sensor 152. The pressuredifferential is a basis for indexing an empirically determined functionthat describes flow through low pressure EGR valve 80. The mass flowthrough low pressure EGR passage 81 is multiplied by the NOxconcentration to provide NOx mass flow rate through low pressure EGRpassage 81. The NOx concentration in the low pressure EGR loop may bedetermined by NOx sensor 127 shown in FIG. 1. The NOx mass flow rates ineach of the respective EGR loops may be determined by multiplying theNOx concentration in the EGR passage by the EGR flow rate through theEGR passage. Method 400 proceeds to 408 after the NOx concentration andNOx flow rates are determined.

At 408, method 400 judges whether or not the NOx concentration and/orthe NOx flow rate in the high pressure EGR loop is greater than athreshold NOx concentration or NOx flow rate. The threshold NOxconcentration and/or NOx flow rate may vary with operating conditions.If the NOx concentration or NOx flow rate in the high pressure EGRpassage is greater than a threshold amount, the mass flow rate of EGR inthe high pressure EGR passage may be increased to attempt to reduce thecylinder combustion temperature. If the NOx concentration or NOx flowrate in the high pressure EGR passage is less than a threshold amount,the mass flow rate of EGR in the high pressure EGR passage may bedecreased to attempt to increase the engine fuel economy. If method 400judges that the NOx concentration and/or NOx mass flow rate in the highpressure EGR loop is greater than desired, method 400 proceeds to 410.Otherwise, method 400 proceeds to 412.

At 410, method 400 increases the mass flow rate in the high pressure EGRpassage to provide additional EGR to the engine. However, the mass flowrate in the high pressure EGR passage may also be limited to a thresholdamount. The EGR mass flow rate in the high pressure EGR passage may beincrementally increased so that the high pressure EGR mass flow rategradually increases. The EGR flow rate in the low pressure EGR loop maybe decreased to offset the increase in the EGR flow rate of the highpressure EGR loop. Method 400 proceeds to 412 after the mass flow ratein the high pressure EGR passage is increased.

At 412, method 400 judges whether or not the NOx concentration and/orthe NOx flow rate in the low pressure EGR loop is greater than athreshold NOx concentration or NOx flow rate. The threshold NOxconcentration and/or NOx flow rate may vary with operating conditions.If the NOx concentration or NOx flow rate in the low pressure EGRpassage is greater than a threshold amount, the mass flow rate of EGR inthe high pressure EGR passage may be increased to attempt to reduce thecylinder combustion temperature. If the NOx concentration or NOx flowrate in the low pressure EGR passage is less than a threshold amount,the mass flow rate of EGR in the high pressure EGR passage may bedecreased to attempt to increase the engine fuel economy. If method 400judges that the NOx concentration and/or NOx mass flow rate in the lowpressure EGR loop is greater than desired, method 400 proceeds to 414.Otherwise, method 400 proceeds to exit. If method 400 exits, it may alsoreduce the EGR mass flow rate in the high pressure EGR loop to attemptto improve engine fuel economy.

At 414, method 400 adjusts the EGR mass flow rate in the high pressureEGR loop in response to the NOx concentration or NOx flow rate in thelow pressure EGR loop. The EGR mass flow rate in the high pressure EGRloop may be increased via further opening the high pressure EGR valve.The EGR flow rate in the low pressure EGR loop may be decreased tooffset the increase in the EGR flow rate of the high pressure EGR loop.Method 400 proceeds to exit after the high pressure EGR flow rate isadjusted.

Additionally, a ratio of low pressure EGR to high pressure EGR may beadjusted while the total EGR amount provided to the engine is maintainedin response to a NOx concentration in the low and/or high pressure EGRloop passages. If NOx concentration in the low pressure EGR passage in aparticular SCR/LNT temperature range is greater than desired, the massfraction of EGR entering the engine from the high pressure EGR loop maybe increased. Similarly, if NOx concentration in the low pressure EGRpassage in a particular SCR/LNT temperature range is less than desired,the mass fraction of EGR entering the engine from the high pressure EGRloop may be decreased.

Thus, the method of FIG. 4 provides for a method for operating anengine, comprising: adjusting an amount of high pressure EGR and anamount of low pressure EGR supplied to an engine in response to a NOxmass flow in one of a high pressure EGR passage and a low pressure EGRpassage. The method includes where an EGR flow rate in the high pressureEGR passage is greater than an EGR flow rate in the low pressure EGRpassage when a NOx concentration in the low pressure EGR passage isgreater than a threshold NOx concentration. The method also includeswhere an EGR flow rate in the high pressure EGR passage is less than anEGR flow rate in the low pressure EGR passage when a NOx concentrationin the low pressure EGR passage is less than a threshold NOxconcentration. The method includes where the amount of high pressure EGRand the amount of low pressure EGR combine in the engine to provide adesired amount of EGR. The method includes where the amount of highpressure EGR is increased in response to an increase in NOxconcentration in the low pressure EGR passage. The method includes wherethe amount of high pressure EGR is decreased in response to a decreasein NOx concentration in the low pressure EGR passage.

As will be appreciated by one of ordinary skill in the art, the methodsdescribed in FIGS. 3 and 4 may represent one or more of any number ofprocessing strategies such as event-driven, interrupt-driven,multi-tasking, multi-threading, and the like. As such, various steps orfunctions illustrated may be performed in the sequence illustrated, inparallel, or in some cases omitted. Likewise, the order of processing isnot necessarily required to achieve the objects, features, andadvantages described herein, but is provided for ease of illustrationand description. Although not explicitly illustrated, one of ordinaryskill in the art will recognize that one or more of the illustratedsteps, methods, or functions may be repeatedly performed depending onthe particular strategy being used.

This concludes the description. The reading of it by those skilled inthe art would bring to mind many alterations and modifications withoutdeparting from the spirit and the scope of the description. For example,single cylinder, I2, I3, I4, I5, V6, V8, V10, V12 and V16 enginesoperating in natural gas, gasoline, diesel, or alternative fuelconfigurations could use the present description to advantage.

The invention claimed is:
 1. A method for operating an engine,comprising: adjusting an actuator in response to a NOx mass flow rate ina low pressure EGR passage between an engine exhaust system and anengine air intake system, where the NOx mass flow rate is estimatedbased on a concentration of NOx downstream of a SCR catalyst and a massflow rate in the low pressure EGR passage.
 2. The method of claim 1,where the actuator is an EGR valve, and further comprising reducing anamount of EGR supplied to the engine in response to a reduction in NOxmass flow rate in the low pressure EGR passage.
 3. A method foroperating an engine, comprising: adjusting an actuator in response to aNOx mass flow rate in a low pressure EGR passage between an engineexhaust system and an engine air intake system, where the actuator is aurea injector, and further comprising decreasing an amount of ureainjected to the engine exhaust system as the NOx mass flow ratedecreases.
 4. The method of claim 3, where the NOx mass flow rate isestimated based on a concentration of NOx downstream of a SCR catalystand a mass flow rate in the low pressure EGR passage.
 5. A method foroperating an engine, comprising: adjusting an actuator in response to aNOx mass flow rate in a low pressure EGR passage between an engineexhaust system and an engine air intake system, where the actuator is afuel injector, and further comprising advancing a start of fuelinjection timing in response to a reduction in NOx mass flow rate in thelow pressure EGR passage.
 6. A method for operating an engine,comprising: adjusting an actuator in response to a NOx mass flow rate ina low pressure EGR passage between an engine exhaust system and anengine air intake system, where the actuator is a fuel pump, and furthercomprising reducing fuel pump output pressure in response to a reductionin NOx mass flow rate in the low pressure EGR passage.
 7. A method foroperating an engine, comprising: adjusting an actuator in response to aNOx mass flow rate in a low pressure EGR passage between an engineexhaust system and an engine air intake system, where the actuator is aturbocharger waste gate or variable geometry turbocharger actuator, andfurther comprising reducing boost pressure in response to a reduction inNOx mass flow rate in the low pressure EGR passage.
 8. A method foroperating an engine, comprising: adjusting an amount of high pressureEGR and an amount of low pressure EGR provided to an engine in responseto a NOx mass flow in one of a high pressure EGR passage and a lowpressure EGR passage, where the amount of high pressure EGR is increasedin response to an increase in NOx concentration in the low pressure EGRpassage.
 9. The method of claim 8, where an EGR flow rate in the highpressure EGR passage is greater than an EGR flow rate in the lowpressure EGR passage when a NOx concentration in the low pressure EGRpassage is greater than a threshold NOx concentration.
 10. The method ofclaim 8, where an EGR flow rate in the high pressure EGR passage is lessthan an EGR flow rate in the low pressure EGR passage when a NOxconcentration in the low pressure EGR passage is less than a thresholdNOx concentration.
 11. The method of claim 8, where the amount of highpressure EGR and the amount of low pressure EGR combine in the engine toprovide a desired amount of EGR, and wherein a NOx mass flow rate in thehigh pressure EGR passage is based on a NOx concentration in the highpressure EGR passage, and where the NOx concentration in the highpressure EGR passage is based on estimated SCR efficiency or a known NOxconcentration when an SCR is operating at less than a thresholdefficiency.
 12. The method of claim 8, where the amount of high pressureEGR is decreased in response to a decrease in NOx concentration in thelow pressure EGR passage.